Evolution stones as you know them (water, fire, etc.) don't exist in the current version of Pokémon GO. By default, when you evolve an Eevee in Pokémon GO, the evolution is chosen randomly from the first set of potential "eeveelutions" (Vaporeon, Jolteon, Flareon).
To get other Eeveelutions like Espeon/Umbreon, other requirements must be met. See @MBorg's answer for more details.
There is also a method for forcing a specific evolution by giving your Eevee a certain nickname before it evolves. See @Brant's answer for more details.
Yes, you still need to feed your Eevee candies to evolve it. The trick you're referring to is a popular trick to control which Pokémon Eevee will evolve into.
In the traditional games, you need to give Eevee an evolution stone to make it evolve. A Water Stone for Vaporeon, a Thunder Stone for Jolteon, and a Fire Stone for Flareon. Since these stones don't exist in Pokémon Go, Eevee evolves randomly into either.
To avoid having to rely on luck, people have attempted to determine what Eevee will evolve into. Eventually, it was discovered that renaming Eevee allows players to define which Pokémon Eevee will evolve into, assuming that evolution had not been registered in their Pokédex before:
Evolution |
Eevee Nickname |
Regular Evolution method |
Vaporeon |
Rainer |
Chosen at random if no other evolution requirement is met |
Jolteon |
Sparky |
Chosen at random if no other evolution requirement is met |
Flareon |
Pyro |
Chosen at random if not other evolution requirement is met |
Espeon |
Sakura |
Walk 10km with Eevee as your Buddy, and evolve in the day |
Umbreon |
Tamao |
Walk 10km with Eevee as your Buddy, and evolve at night |
Leafeon |
Linnea |
Evolve near Pokéstop under the effect of Mossy Lure |
Glaceon |
Rea |
Evolve near Pokéstop under the effect of Glacial Lure |
Restarting the game is only used to ensure that your Eevee has been renamed, since the servers aren't always available. By restarting, you can determine if your Eevee has been successfully renamed before you spend candies to evolve it.
Of course, this doesn't change the fact that you still need to candies to evolve your Pokémon. Also keep in mind that the trick only works if you haven't registered the respective evolution in your Pokédex yet.
Best Answer
If you only want to evolve Umbreon for the Pokédex entry, the easiest way is to rename your Eevee to Tamao and it will 100% evolve into Umbreon without needing any of the other requirements. Note that this only works once. This is possible once-per Eeveelution, see this answer for all possible names that work.
Here are also a couple of other nuggets that will be useful if you want to get an Umbreon the normal way: https://www.reddit.com/r/TheSilphRoad/comments/96o6nu/psa_the_umbreonespeon_method_is_walk_10k_for_two/
But personally, if you want Umbreon for Pokédex filling purposes, just do the name method. I did this myself a few years back and it worked, but as I said, it only works once.